Conventionally, soft magnetic materials are widely used in current transformers, magnetic head transformers, choke coils, current transformers and other applications due to the materials' high magnetic flux density, high magnetic permeability and low energy expense or low core loss. Traditionally, a variety of crystalline soft magnetic alloys have been used in the applications mentioned above; these include the alloy PERMALLOY, ferrites (magnetic oxides) and iron-silicon steel. In recent years, however, there is an increasing demand for improved electronic equipment with higher operating efficiency under high frequencies and/or high temperatures. Consequently, there is a growing desire for magnetic materials that constitute magnetic parts with improved properties such as low core loss, high saturation magnetic flux density, high Curie temperature, linear magnetization as a function of field, and the like in the high frequency region.
Existing soft magnetic materials, as mentioned above, however, cannot satisfy these new requirements due to the nature of their crystalline structure. Thus, amorphous alloys have recently attracted attention because they exhibit excellent soft magnetic properties such as high permeability, low coercive force and the like. Amorphous alloys also have the properties of low core loss, high squareness ratio and the like at high frequency. Because of these advantages, some amorphous alloys have been put to practical use as the magnetic material for switching power supplies. Furthermore, amorphous alloys can also be transverse field heat treated to produce so-called flat loop materials with constant permeabilities, properties that are highly desirable in applications such as current transformers.
In previous attempts to advance transformer technology, amorphous magnetic alloys having a high saturation magnetic flux density and low core loss have been investigated. Such amorphous magnetic alloys are typically base alloys of Fe, Co, Ni, etc., and contain metalloids as elements promoting the amorphous state, (P, C, B, Si, Al, and Ge, etc.). For example, U.S. Pat. No. 5,160,379 to Yoshizawa et al. discloses an alloy for a transformer having a high saturation magnetic flux density and exhibiting a low core loss has been disclosed in The composition of the Yoshizawa alloy is expressed by the general formula:(Fe1-αMα)100-x-y-z-α-β-γCuxSiyBzM′αM″βXγwhere M is Co and/or Ni; M′ is at least one element selected from the group consisting of Nb, W, Ta, Zr, Hf, Ti and Mo; M″ is at least one element selected from the group consisting of V, Ti, M, Al, elements in the platinum group, Sc, Y, rare earth elements, Au, Zn, Sn and Re; X is at least one element selected from the group consisting of C, Ge, P, Ga, Sb, In, Be and As; and a, x, y, z, α, β and γ respectively satisfy 0≦a≦0.5, 0.1≦x≦0.3, 0≦y≦30, 0≦z≦25, 5≦y+z≦30, 0.1≦α≦30, β≦10 and γ≦10, with at least 50% of the alloy structure being occupied by fine crystalline particles having an average particle size of 1,000 Å (100 nm) or less. Yoshizawa further teaches that the properties of its alloy may be further modified by field heat treating; however, the strength and temperature stability of the beneficial pair induced anisotropy of the Yoshizawa alloy is limited as compared to alloys which contain more Co. The Yoshizawa alloy is also limited by its lower induction and Curie temperature.
Yoshizawa also teaches that the omission of Cu cannot easily produce fine crystalline grains, causing a compound phase that lacks the desired magnetic characteristics. It is thereby necessary for the alloy of the foregoing type disclosed by Yoshizawa to contain Cu because the addition of Cu causes fluctuations to occur in the local composition in the amorphous state, generating desirable fine crystalline grains. However, the necessary addition of non-magnetic Cu is a limitation of this alloy because it reduces the overall magnetic strength of the material.
Another similar alloy called NANOPERM, based on Fe(Co,Ni)—Zr alloy system, is disclosed in U.S. Pat. No. 5,474,624 to Suzuki et al. The general composition of NANOPERM can be expressed by:(Fe1-aZa)bBxMyTzXu where Z is Co and/or Ni; M is one or more elements selected from a group consisting of Ti, Zr, Hf, V, Nb, Ta, Mo and W and contains Zr and/or Hf; T is one or more elements selected from a group consisting of Cu, Ag, Au, Pd, Pt and Bi; X is one or more elements selected from a group consisting of Cr, Ru, Rh and Ir, a≦0.1 atomic %, 75≦b≦93 atom %, 0.5≦x≦18 atom %, 4≦y≦10 atom %, z≦4.5 atom % and u≦5 atom %. The NANOPERM alloy has limitations in the magnitude of its saturation induction and low Curie temperatures.
Other kinds of FeCo based nanocomposite soft magnetic alloys have been developed, such as an Fe-M-B alloy system that was disclosed in U.S. Pat. No. 6,284,061 to Inoue et al. The Inoue alloy has the general composition formula:(Fe1-a-bCoaNib)100-a-bMxByTz wherein 0≦a≦0.29, 0≦b≦0.43, 5 atomic %≦x≦20 atomic %, 10 atomic %≦y≦22 atomic %, and T is at least one element of Cr, W, Ru, Rh, Pd, Os, Ir, Pt, Al, Si, Ge, C and P; and M is at least one element of Zr, Nb, Ta, Hf, Mo, Ti and V. However, it is necessary that the content of M of the Inoue alloy system is over 5 atomic % and the value of “a” (Co content) is below 0.3. Both of these limit the ultimate induction.
In U.S. Patent Application Publication No. US 2006/0077030 by Herzer et al., an alloy of the composition FeaCobNicCudMeSifBgXh was disclosed, wherein M represents at least one of the elements V, Nb, Ta, Ti, Mo, W, Zr, Cr, Mn, and Hf; a, b, c, d, e, f, g and h indicate atomic percent; X represents the elements P, Ge, C and commercially available impurities; and a, b, c, d, e, f, g and h satisfy the following conditions: 0≦b≦40; 2<c<20; 0.5≦d≦2; 1≦e≦6; 6.5≦f≦18; 5≦g≦14; h<5 atomic %; 5≦b+c≦45, and a+b+c+d+e+f=100, and a is seen to be the balance of the constituents. The content of Si in this alloy must be higher than 6 atomic %. This alloy is limited by its high Si content which reduces both the induction and Curie temperatures.
U.S. Patent Application Publication No. 2006/0118207 by Yoshizawa disclosed the alloy (Fe1-aCoa)100-y-cM′yX′c(atomic %), where M′ represents at least one element selected from V, Ti, Zr, Nb, Mo, Hf, Ta, and W; X′ represents Si and B, an Si content (atomic %) is smaller than a B content (atomic %), the B content is from 4 to 12 atomic %, and the Si content is from 0.01 to 5 atomic %, a, y, and c satisfy respectively 0.2<a<0.6, 6.5≦y≦15, 2≦c≦15, and 7≦(y+c)≦20. The content of M′(such as Nb) is above 6 atomic %. This alloy is limited by its high M′ (Nb) content which reduces both the induction and Curie temperatures.
A group at Carnegie Mellon University (Amorphous and Nanocrystalline Materials for Applications as Soft Magnets. M. E. McHenry, M. A. Willard and D. E. Laughlin; Prog. Mat. Sci., 44, 291, (1999)) attempted to enhance Curie temperature by adding Co into Fe-base alloys to form a new alloy called HITPERM. These materials exhibited losses that were too high for the applications described above.
When core materials are to be used in high-accuracy current transformers, such as those used in domestic power meters, additional concerns arise. To produce a current transformer with high accuracy, it should be made on a base of high permeability and high saturation flux density magnetic core material. The most conventional magnetic materials for this application are silicon steel, Ni-base permalloy and Fe-base nanocrystalline alloy. However, these are unsuitable for use in domestic meters because modern semiconductor circuits, such as rectifier circuits or phase-angle circuits, create current flows that are not symmetrical about the zero applied field and contain direct current components. This magnetically saturates the current transformer and thus falsifies the power reading. Accordingly, core materials having a relatively low permeability and a linear hysteresis loop are desirable.
Vacuumschmelze GmbH & Co. of Hanau, Germany has attempted to address this problem by developing an amorphous Co-based magnetic alloy known as VITROVAC6150. VITROVAC6150 (also referred to herein as “VAC6150”) has a relatively low permeability (around 1500) and extremely linear hysteresis loop. Accordingly, current transformers utilizing a core made from this material do not go to saturation in presence of a typical direct current component. VITROVAC6150 transformers do have rather high phase and amplitude errors, but because of constant value of permeability, the errors values are constant as well and can be easily eliminated during the calculation of power. Also, the VITROVAC6150 material has rather low saturation flux density (around 1 T) and a high cost because of its high cobalt content.